Each time at the well produces a 50/50 chance. one sperm in a billion getting through. However if the sire has allready produced one girl the likelihood of a repeat is higher than that of producing a boy. Atheletic types have a higher temperature and predominantly produce girls. There is all sorts of other criteria which may slightly change the odds.

A bit like horse racing.

Besides that I hope everybody had a good Christmas and has a prosperous New Year